To produce various transfer functions, suitable filters are used. In this case, a distinction is drawn between active, i.e. analog, filters and digital filters. In the case of analog filter voltages, the processed signals are present in the form of continuous time functions. The functionality of an analog filter may also be provided in digital form. On account of the additional circuit complexity required for this and the fact that, particularly with a large number of data items to be processed, the analog/digital conversion and the digital/analog conversion are also time consuming, analog, continuous-time filters are used, particularly for very fast applications.
One special form of such an analog, continuous-time filter has a forward-amplifying architecture. Such forward-amplifying filter circuits, which are also frequently referred to as feedforward filters in technical literature, are known generally and are distinguished particularly by their performance. Forward-amplifying filter architectures are used particularly in telecommunications, for example in mobile radio and for broadband applications.
FIG. 1 uses a schematic block diagram to show the general design of a fifth-order forward-amplifying filter circuit. Such a filter generally comprises poles and zero points, the poles being produced by integrators a1–a5 and the zero points being produced by coefficients c1–c5. The coefficients c1–c5 are formed by forward-amplifying paths in FIG. 1. In addition, feedback paths with coefficients d1, d2 are provided. At the output of the forward-amplifying filter circuit, there is a summing node, which means that an analog output signal Vout can be provided at the output from the analog input signal Vin which is input on the input side.
FIG. 2 shows a known circuit implementation for the forward-amplifying filter circuit from FIG. 1. In this case, the integrators a1–a5 have been produced by operational amplifiers OP1–OP5, resistors R1–R5 and capacitances C1–C5, and the feedforward coefficients c1–c5 have been produced by resistors RK1–RK5. To produce the summing node at the output of the filter circuit, however, an additional summing amplifier OPout with a parallel-connected resistor Rout is required. However, an additional operational amplifier OPout not only increases the power consumption of the overall filter circuit but also means an increase in execution time. Although it would also be possible to dispense with the summing amplifier OPout at the output of the filter circuit, the filter circuit would no longer operate in isolation in this case.
As an alternative to the implementation in FIG. 2, the integrators a1–a5 would also be able to be formed by transconductance amplifiers (OTA, voltage/current amplifiers), and the feedforward coefficients would be able to be formed by transconductance amplifiers or capacitances. The transconductance amplifiers or capacitances which such a filter circuit requires in the forward-amplifying paths are synonymous with nonlinear forward amplification, however, which means that use is preferably made of filter circuits in which the integrators are produced by operational amplifiers, resistors and capacitances, as shown in FIG. 2.
In such power-optimized filter circuits, the number of poles should be as equal as possible to the number of amplifying components.